Zinc-containing plated steel materials and other plated steel materials are being widely used as building materials and members of automobiles, household electrical appliances, etc. due to the high corrosion preventing function of the plating layer metal. However, in plated steel materials, the phenomenon sometimes arises of oxidation due to the salts and other electrolytes contained in the air and the oxygen and moisture present in high temperature, humid environments and consequent formation of white rust and corrosion. Further, in certain environments of a high temperature and humidity, the phenomenon arises of the plated steel material discoloring and appearing to be black. Both these phenomena are due to the deterioration of the plating layer metal and are sometimes considered a problem from the viewpoint of quality and aesthetics when the material is assembled into the above various products.
Further, even when used painted, the penetration of oxygen or moisture sometimes leads to the peeling of the paint film due to the formation or buildup of corrosion products at the plating layer under the paint film. Sometimes a problem is caused in terms of aesthetics and practical use.
Further, sometimes a plated steel material is cleaned by an alkaline degreasing agent after being shaped. In this case, if the material is not durable against alkali, it will discolor or will end up corroding early during use.
As means for preventing such corrosion, blackening, or paint peeling of plated steel materials, in the past the surfaces have been treated by various techniques bringing chrome-containing treatment solutions, such as chromic acid-chromate or phosphoric acid-chromate solutions, into contact with the surfaces of the plated steel materials. By forming what is generally called a “reaction type chromate coating” on the surface of a plated steel material, the above problems are avoided. The chromate coatings obtained by these treatments are mainly comprised of trivalent chrome. While the amount of leaching of the particularly toxic hexavalent chrome is small, the corrosion prevention property cannot be said to be sufficient. In particular, when the damage to a coating due to shaping or scratches reaching the base iron is large, the plated steel material drops in corrosion resistance.
On the other hand, in coating type chromate treatment where a treatment solution containing hexavalent chrome is coated by a roll coater etc. on a material and dried, the coating formed will contain a large amount of hexavalent chrome. Therefore, even if the chromate coating is damaged due to being worked or scratches etc., the material will have superior corrosion resistance, but sometimes hexavalent chrome will leach out from the chromate coating. Chromate treatment coatings containing hexavalent chrome have a detrimental effect on the human body upon buildup due to their toxicity. As explained above, the coating easily leaches out by nature. Therefore, there can be said to be the problem in terms of environmental protection of the environmental load substances moving outside the system.
In this way, the chromate treatment performed on plated steel materials in the past for the purpose of preventing the formation of white rust is becoming an issue in terms of safety and environmental impact due to the hexavalent chrome. To solve this problem, technology for replacing chromate treatment has been studied.
As a publication describing art replacing chromate by coating a chromate-free treatment solution on the surface of a plated steel material, JP 2002-332574 A may be mentioned. This publication proposes the art of coating a treatment solution containing zirconium carbonate complex ions and vanadyl ions, dimercaptosuccinic acid, etc. and heating it to dry to form a coating of a dense three-dimensional structure and obtain a superior corrosion resistance due to the high ability of adsorption on the surface metal. Further, JP 2002-030460 A describes metal surface treatment agents containing a vanadium compound and a compound containing at least one metal selected from zirconium, titanium, molybdenum, tungsten, manganese, and cerium and metal surface-treated metal materials. Further, JP No. 2004-183015 A describes metal surface treatment agents containing a vanadium compound and a metal compound containing at least one metal selected from cobalt, nickel, zinc, magnesium, aluminum, etc. and metal surface-treated materials.